As an ordinary flow rate measuring device of this kind, a device utilizing a sing-around method is introduced, in which a signal transmission and reception are repeated by two transducers multiple times for enhancing measuring resolution. This type of flow rate measuring device applied to a home gas meter will be explained as an example by using FIG. 7.
FIG. 7 is a block diagram of a conventional flow rate measuring device utilizing the sing-around method. As in FIG. 7, the device is composed of first transducer 32 for transmitting an ultrasonic wave signal and second transducer 33 for receiving the ultrasonic signal placed in a middle of fluid pipe 31 and oppositely facing a flowing direction. The flow rate measuring device is also composed of measuring part 34 for measuring a propagation time of the ultrasonic signal with transducers 32 and 33, controlling part 35 for controlling measuring part 34, and calculating part 36 determining a flow rate of fluid based on measurement of measuring part 34.
In FIG. 7, assume an acoustic sound velocity is C, a flow velocity is v, a distance between the two transducers is L, and an angle between a propagation direction and a flow direction is θ. When first transducer 32 at an upper stream of the fluid pipe transmits an ultrasonic signal and second transducer 33 at a lower stream receives the signal, assume a propagation time of the signal is t1, and a propagation time in a reverse direction t2, then t1 and t2 are each expressed by the following formula.t1=L/(C+v cos θ)  (1)t2=L/(C−v cos θ)  (2)Formula (1) and (2) may be transformed into the formula (3), obtaining flow velocity v.v=L(1/t1−1/t2)/2 cos θ  (3)Multiplying v obtained in (3) by a cross section S of fluid pipe 32, a flow rate of the fluid is obtained.
The items in the parenthesis in formula (3) may be transformed into the following formula (4)(t2−t1)/t1t2  (4)
Generally, in this kind of flow rate measuring device, flow velocity v is far smaller than an acoustic sound velocity C. It is also known that acoustic sound velocity C is approximately expressed by a linear function of temperature. However, assuming that there is no abrupt change in temperature in a short period of time, acoustic velocity C may be considered to be a constant value. The denominator in formula (4) is therefore essentially constant regardless of change in flow velocity, meaning the item in the numerator is almost proportioned to the flow velocity. So, in order to obtain an accurate flow velocity v, the propagation time difference between the two ultrasonic waves needs to be measured accurately. As flow velocity becomes slower, much finer time difference between the two propagation times needs to be measured. So, for measuring a single phenomenon, measuring part 34 must have a very high time resolution, such as in ns order. Such a high time resolution is hardly obtained. Even if it is obtained, power consumption is inevitably increased for raising the resolution. For that reason, ultrasonic wave signal transmission is repeated and measuring part 34 measures time required for the repeated measurement. By averaging the values, a necessary time resolution is realized. Assume a time resolution of measuring part 34 is TA and a repetition times is M, a measuring resolution of propagation time is obtained by TA/M as long as measuring part 34 is continually operates the repeated measurement,
This type of flow rate measuring device realizes a high precision measurement when a pressure inside the path of fluid is stabilized. But, when the device is applied to a gas meter measuring a flow rate of household gas, it faces an inherent problem called “pulsation”. This phenomenon is seen in a gas-engine driven air-conditioner or a GHP (gas heat pump), for example, where a pressure inside a gas pipe around the equipment is varied synchronized with rotation of the engine. If the pulsation occurs, gas flows moving inside the pipe synchronized with the change in pressure even when the appliance is not used, outputting a measurement value as if the gas is flown inside.
For controlling the adverse effect of this phenomenon, a method is proposed such as in Unexamined Japanese Patent Publication No. 2002-35202. In this method, repetition times M of measurement is controlled to a minimum possible level while maintaining a certain measuring precision. With this condition, a measurement interval is shortened and measurement is conducted frequently for relatively a long time. The flow rate is calculated using the consecutive measurements. By making a measurement interval sufficiently shorter than a cycle of the pressure change, a changing phase status in a flow velocity waveform is evenly captured. So, by averaging the consecutive measurement, a net flow velocity (flow rate) eliminated of a varying component may be obtained.
FIG. 8 an example using the method, in which repetition times M is 4. In FIG. 8, a time T1 is a propagation time from an upstream to a downstream and a time T2 is a propagation time from the downstream to the upstream. Switching the transmitting and receiving direction, measurement is continued 20 times each, total T40. A sum total of each propagation time is obtained and averaged to determine a propagation time in each direction, therewith a flow rate is determined. With this method, the flow velocity is averaged whether the pulsation exists or not, therewith an accurate flow rate is obtained. For this reason, a proposal using this method is increasing in number recent years.
A home gas meter is equipped with a security function as well as a function as a meter. In recent years, gas meter is requested to follow a change in a flow rate in a short interval (less than a second, 0.2 sec for instance) thus to enhance security function. However, with a conventional flow rate measuring device as in above, the flow velocity is averaged for relatively a long period of time (2 sec., for instance) then is converted into a flow rate, so a change in a momentary flow rate is not detectable.